Tracing IIS 500 Error Using Failed Request Tracing Rules

After extended a SharePoint Web Application I got a 500 server error. There's not related event log and the IIS log shows:

2009-12-02 19:37:21 IP GET / - 10.10.1.11 Jakarta+Commons-HttpClient/3.1 500 19 183 0

The HTTP status code 500.19 for IIS 7.0 is "Configuration data is Invalid.". That's not very helpful and I couldn't find obvious issue in web.config. To see more detailed error message I enabled IIS Failed Request Tracing, where I was able to find out the exact error in configuration file:



So the error is caused by "cannot add duplicate collection entry of type 'add' with unique key attribute 'name' set to 'session'". It looks like the "Session" module has already been registered somewhere by SharePoint when the configuration entries are merged. Change the the web.config from original:

    <add name="Session" type="System.Web.SessionState.SessionStateModule" />
    <remove name="Session" />

to:

    <remove name="Session" />
    <add name="Session" type="System.Web.SessionState.SessionStateModule" />

Then the extended web application works normally.